1. Technical Field
The invention relates to devices for electrically heating a medium by resistance, which devices are further commonly called immersion heaters, of the type comprising a substantially cylindrical elongate graphite heating element, surrounded by a refractory material sheath defining, about the heating element, a sealed chamber retaining a gas capable of protecting the graphite from oxidation.
It finds a particularly important, although not exclusive, application in the field of electric heating devices reaching, in furnaces where a corrosive or oxidizing atmosphere prevails, temperatures as high as those reached by the use of a fossil fuel.
2. Electric heating prior art devices of the above defined type are already known. In particular, in the patent application FR-A-84 02358, the applicant describes a device comprising a graphite heating element protected against oxidation by an atmosphere containing carbon monoxide formed from the heating element when the device is first put into use and surrounded by a corrosion resistant tubular refractory ceramic sheath. This solution solves numerous problems.
However, the use of a ceramic sheath, whose heat conductivity is low, limits the power which can be dissipated per unit of surface.